Gas separation



Oct. 2l, 1958 w. R. IDAR'lfRiDc'ala ETAL 2,857,018

GAS SEPARATION Filed Sept. 27, 1956 ATTORNEY United States Patent GAS SEPARATION William Russell Partridge, Livingston, and Frederick Louis Mahoney, Westfield, N. J., assignors to The M. W. Kellogg Company, Jersey City, N. i., a corporation of Delaware Application September 27, 1956, Serial No. 612,478

' 8 Claims. *(Cl. 18S-115) This invention relates to the separation of C2 hydrocarbons from gaseous mixtures containing thersame. One yaspect of this invention relates to an improved distillationabsorption process for the recovery of a fraction rich in C2 hydrocarbons from a gaseous mixture containing the same. Another aspect of this invention relates to the use of such a process in the recovery of ethylene.

Gases containing substantial quantities of C2 hydrocarbons are obtained from many processes involving treating or conversion of hydrocarbons such as catalytic cracking, naphtha reforming, vdesulfurization and many other thermal and catalytic processes. These gases frequently contain, in addition to C2 hydrocarbons, other lighter and heavier normally gaseous hydrocarbons as well as low boiling gases such as oxygen, nitrogen, carbon monoxide hydrogen, etc. These gases frequently contain substantial quantities of ethylene which itis highly desirable to recover; however, the large amounts of other material present may make it undesirable to process the entire quantity of gas for the recovery of ethylene by successive distillation steps in the usual manner. In such cases, it may be desirable to treat the gaseous mixture in question for the separation of a fraction rich in C2 hydrocarbons from which the major portion of the lighter hydrocarbons and other lower boiling components has been removed. The C2-rich fraction can then be further treated by conventional distillation processes for the recovery of ethylene of whatever purity may be desired. Natural gas may also be treated according to this invention. Any of the conventional methods for recovering ethylene may be used for recovering ethylene from the C2-rich fraction. If the C2-rich fraction is not subjected to an ethylene recovery process, it may be used or disposed of in any other desired manner. Heavier hydrocarbons and other materials such as CO2 or -I-I2S which may be present in the gaseous feed mixture may be removed separately if desired but this may be unnecessary in view of the relatively small quantities of these various materials involved. These materials may sometimes be removed during the treatment of the C2-rich fraction for recovery of ethylene.

It is an object of this invention to provide an improved vC2 hydrocarbons in preference to methane.

ice

mally gaseous mixture containing the same, methane and inert gases.

Other objects and advantages of this invention will become apparent to those skilled in the art from the follow- 'ing description and disclosure. e

According to one aspect of this invention, a Agaseous feed mixture containing methane and C2 hydrocarbons is passed to a first absorptlon zone in which it is contacted with a first absorption medium which is adapted to absorb This results in the formation in the first absorption zone of a liquid phase containing absorption medium and substantiallyall of the C2 hydrocarbons and a vaporous phase containing methane and entrained absorption medium. The vaporous phase from the first absorption Zone is withdrawn and passed to a second absorption zonewhere it is contacted with a second absorption medium adapted to preferentially absorb the first absorption medium. 'Ihis results in the formation, in the second absorption zone, of a vapor phase substantially free of both iirst andsecond absorption mediums and a liquid phase comprising said rst and second absorption mediums. The liquid and vapor phases thus formed are separately Withdrawn from the second absorption zone. The liquid and Vapor phases Withdrawn from the second absorption zone may be treated or disposed of in any desired manner; however, it is usually 'advantageous to treat the liquid from the second absorption zone for recovery of the rst and second absorbents so that they may be used again, if desired. The liquid phase in the rst absorption zone containing the first absorption medium and substantially all of the C2 hydrocarbons is withdrawn and passed to a distillation zone where it is fractionated into a vaporous overhead fraction preferably containing substantially all of the C2 hydrocarbons and a liquid bottoms fraction comprising said iirst absorption medium. The bottoms fraction comprising the first absorption medium is withdrawn from the distillation zone and passed to the first absorption zone While the overhead fraction from the distillation zone is withdrawn as a product of the process.

The normally gaseous feed mixture used in practicing this invention contains C2 hydrocarbons including ethylene and also contains methane. In addition, the feed mixture may contain other normally gaseous materials such as C3 and C., hydrocarbons, hydrogen, H28, CO2, oxygen, nitrogen, carbon monoxide, etc. These gases Y may be found in varying quantities in the feed mixture;

process for the separation of normally gaseous hydrocarbons.

It is another object of this invention to provide an irnproved process for the separation of C2 hydrocarbons from gaseous mixtures containing the same and methane.

Another object of this invention is to provide an improved process for the recovery of an ethylene-rich fraction from a gaseous mixture containing the same.

A further object of this invention is to provide an improved absorpton-distillation process for the recovery of a fraction rich in C2 hydrocarbons from a gaseous mixture containing the same.

It is another object of this invention to provide an irnproved absorption-distillation process for the recovery of a fraction rich in C2 and C3 hydrocarbons from a norhowever, this invention is particularly applicable where the relative quantity of C2 hydrocarbons present is appreciable but does not constitute a majority of the gaseous materials in the mixture. Gases, such as tail gases from catalytic cracking processes, which contain appreciable quantities of ethane and ethylene `but in which there are also large quantities of other gases such as oxygen, nitrogen, carbon monoxide and methane are particularly well suited for use in practicing this invention.

Any suitable absorbents may be used in practicing this invention so long as the irst absorbent is capable of absorbing C2 hydrocarbons in preference to methane while the second absorbent is capable of recovering substantially all of the first absorbent which may be entrained in the fraction containing methane which is obtained as a result of treating the feed with the first absorbent. Hydrocarbon absorbents are particularly well adapted for use in accordance with our invention. Light hydrocarbons such as propane, butane or gasoline may be used as the iirst absorbent while the second absorbent may be any hydrocarbon oil of lower volatility than the irst absorbent. Preferably however, the iirstV absorbent is butane while the second absorbent' is a hydrocarbon oil heavier than the rst absorbent. Absorbents lighter than butanes are not entirely satisfactory as `the first absorbent because of the -dilculty ofrpreventing excessive loss of such absorbente with the overhead product of the distillation'zone. Absorbents heavier thanl butanes are not entirely satisfactory as the first absorbent because of the excessive temperatures needed for the separation of such absorbents from the fraction 'containing C2 hydrocarbons in the distillation zone. Generally, it is preferred that the second absorbent Abe as light as possible consistent with proper absorption of the first absorbent and the prevention of excessive loss of either absorbent with the overhead from the second absorption Zone. Hydrocarbon oilshaving an API gravity between about and about are found to be Well suited for use as the second absorbent.

The butanes and hydrocarbon oil which are preferred for use as absorbente ymay conveniently be products of a process for the catalytic cracking of `hydrocarbon oils to produce-gasoline and naphtha fractions although other suitable absorbents may, of course, be used. The -butane and vhydrocarbon oil absorbents maybe obtained from the total product of the cracking process by any suitable means such as distillation7 absorption, or a combination thereof. If the absorbents used are products of such a cracking process, it is frequently both convenient and economical to return the liquid phase from the second absorption zone to a suitable point in the product recovery system of the cracking process so that the first and second absorbents may be recovered. By returning the liquid phase containing the first and second absorbents to the product recovery system, these absorbents can be recovered 'for use as described above or otherwise without the use of any extraneous equipment.

Pressures between about 300 and about 600 p. s. i. g., more usually between about 400 and about 500 p. s. i. g. are preferably maintained in the absorption and distillation zones while practicing our invention. It frequently happens that when low `boiling gases such as oxygen, hydrogen, nitrogen or carbon monoxide are present in the feed gas, a substantial quantity of these gases are entrained by the absorbent in the first absorption zone and are withdrawn from the first absorption zone in the liquid phase. If desired, such entrained gases may be removed by passing the liquid phase from the first absorption zone to a zone of reduced pressure. This zone of reduced pressure is preferably maintained at pressures of at least about p. s. i. below the pressure maintained in the first absorption zone. In this zone of reduced pressure, substantial quantities of entrained low-boiling gases such as oxygen, nitrogen and CO are released from the liquid phase by reason of the reduced pressure and can be withdrawn from the zone of reduced pressure as vapors. The gases so withdrawn may, if desired, be recombined with the feed or may be disposed of in a more direct manner. If such a zone of reduced pressure is used, the liquid phase containing absorbing medium and rich in C2 hydrocarbons may be withdrawn from it and passed to the distillation Zone through a pump which serves to repressurize the liquid phase prior to its entry into the distillation zone.

The first absorption zone is preferably maintained at a temperature of between about 90 and about 120 F. for the liquid phase and between about 90 and about 120 F. for the vapor phase when the preferred pressure conditions are maintained. The second absorption zone is preferably maintained at a temperature between about 100 and about 130 F. for the liquid phase and between about 90 and about 120 F. for the vapor phase when the preferred pressure conditions are maintained. Under the preferred pressure conditions the bottoms temperature of the distillation zone is preferably between about 260 and about 280 F. while the temperature at which the Cz-rich fraction is recovered from the process is preferably between about 50 and about 120 F. If desired, in order to produce reflux for 'the distillation zone conveniently, the overhead fraction may be withdrawn from the distillation zone at a higher temperature than the temperature at which it is desired to recover the Cg-rich fraction. If this is done, the overhead fraction is preferably withdrawn from the distillation zone at a temperature between about 125 and about 135 F. and is then cooled to the temperature at which it is desired to remove the Cz-rich fraction. This further reduction in temperature of the overhead fraction condenses remaining absorbent which may then `be used as reflux to the distillation Zone and allows the Withdrawal of the CZ-rich fraction substantially free of absorbent.

For a better understanding of our invention, reference should be had to the accompanying drawing which is a diagrammatic illustration in elevation of a suitable arrangement of apparatus for carrying out a preferred embodiment of our invention.

in the drawing, feed gas having the composition shown in Table I enters through conduit 11, at the rate of 37,408 lbs/hr.

This feed is the tail gas which remains after the product of a catalytic cracking process has been treated by distillation and absorption for the removal of less volatile constituents. The feed in conduit 11 passes 4through valve12 into a drum 13 where itis combined with low boiling gases from conduit 14 obtained as described bclow. The combining of gases from conduit 14 with .the feed is for convenience only and it is entirely within the scope of our invention to dispose of these gases in any other convenient manner. By recombining the low 'boiling gases from conduit 14 with the feed in drum .13, these gases can be removed from the process, together with other low boiling components of the' feed as hereinafter described. From drum 13, the feedpasses through conduit 16 to a compressor 17 where it is compressed to a pressure of 445 p. s. i. g. and has a resulting temperature of 265 F. Compressor 17 is driven yby a .turbine 18. From compressor 17 feed passes through conduit 19 to a cooler v21 where it is cooled to a temperature of F. The feed then continues from cooler 21 through conduit 19 to an absorber 22, which is an absorption column operated at a pressure of 435 vp. s. i. g. Absorber 22 is maintained at a bottom temperature 108 F. and a top temperature of F. In absorber 22, the feed is contacted with butane absorbent which enters absorber v22 through conduit 23. This butane absorbent comprises butane product from a catalytic cracking process and has a density of 4.90 lb./ gal. The butane absorbent enters the process through conduit 23 and a pump 26 at a temperature of 177 F. at fthe rate of 19,131 lb./hr. The butane absorbent is cooledtoa temperature of 105 F. in a cooler 28 and then'passes via pump 29 to absorber 22 through conduit 23. Additional butane absorbent obtained as explained below enters conduit 23 from conduit 27 so that the total amount of butane absorbent passing into absorber 22 from conduit 23 is 194,559 pounds per hour.

Overhead vapor having the composition shown in Table II is withdrawn from absorber 22 through conduit 31 at the rate of 37,439 pounds per hour and a temperature of 110 F.

TABLE II Composition of vapor phase from absorber 22 Component: Lb./hr. O2N2CO 11,310 CO2 431 H2 551 C1 hydrocarbons 4,882 C2 hydrocarbons 648 C3 hydrocarbons 913 C4 hydrocarbons 18,465 C5 hydrocarbons 163 H2S 6 H2O 70 The vapor in conduit 31 passes to an absorption tower 32 where it is contacted with a second absorbent for the recovery of butanes. Absorber 32 is maintained at a pressure of 430 p. s. i. g. The second absorbent used in this case is a 28.0 A. P..I. light cycle oil which is a product of a catalytic cracking processand has been separated from the total product by distillation. 4The cycle oil enters through conduit33 at the rate of 110,000 pounds per hour and a'temperature of 440 F and passes via pump 34 through a cooler 36 where its temperature is reduced to 221 F. From cooler 36 the cycle oil continues through conduit 33 to a cooler 37y where its temperature is reduced to 105 F. and then continues through conduit 33 to absorber 32. From absorber 32 a vaporous phase iswithdrawn through conduit 38 at a temperature of 108 F. at the rate of 17,441 pounds per hour. The vaporous fraction in conduit 38 comprises gases having lower boiling points than C2 hydrocarbons and has the composition -shown in Table III.

TABLE III Composition of overhead fraction from absorber 32 The vaporous fraction in conduit 38 passes from the process through a valve 39 to be used as fuel gas or to be disposed of in any other suitable manner.

A side stream is withdrawn from the lower portion of absorber 32 through conduit 41 at a temperature of 129 F., passed through a pump 42 to an intercooler 43 where it is cooled to a temperature of 105 F. and is then returned to absorber 32 through conduit 41. Intercooler 43 serves to remove the Aheat of absorption created by the condensation of the butanes which are absorbed by the light cycle oil in absorber 32. Water which is present in absorber 32 is withdrawn through conduit 44 and valve 46 at the rate of 12 pounds per hour. A 41.0 A. P. I. liquid phase comprising butane and light cycle oil absorbents and having the composition shown in Table IV is withdrawn from absorber 32 through conduit 47 at the rate of 129,986 pound per hour at a temperature of 120 F. f

The liquid in conduitl 47 passes through .a pressure reducing valve 48 where it is partially vaporized `and its temperature is reduced to 118 F. and then continues through conduit 47 to heat exchanger 36 where it is heated and further vaporized. From heat exchanger 36 the fluid in conduit 47 continues to a separation drum 49 which is maintained at a pressure of200 p. s. i. g. and a temperature of 300 F. 4Vapor is withdrawn from drum 49 through conduit 51 andpasses from the system through valve 52 while liquid is, withdrawn from drum 49 through conduit 53 and passes from the system through valve 54. The vapor and liquid withdrawn through conduits 51 and 53 may be returned to a suitable point inthe recovery section of the catalytic cracking process, separately treated for recovery of the butanes and light cycle oil or-may be disposed of in any other suitable manner.` Preferably, the vapor and liquid from drum 49 are recombined with `product of the catalytic cracking process at appropriate points in the recovery system so that the various vvaluable components contained therein may be recovered as part of the product fractions of the cracking process, reused as absorbents or otherwise disposed of. The liquid phase from absorber 32 need not, of course, `be treated in the manner described but may be disposed of in anyl other suitable manner without departing from the scope of our invention.

A liquid phase is `withdrawn from absorber 22 through conduit 56 and passes through a pressure, reducing valve 57 to a separating drum 58 which is maintained'at a pressure of 335 p. s. i. g. and'a temperature of 106 F. Water from absorber 22 is withdrawn at a rate of 178 pounds per hour through conduit 59 and valve 61. The liquid phase withdrawn ythrOl'lgll conduit 56 has the composition shown in Table V.

TABLE v Composition of liquid phase from absorber 22 Component: y Lb./ hr. O2N2CO 953 CO2 Y `1,827 H2 61 H2S 1,090 C1 hydrocarbons 2,419 C2 hydrocarbons 8,244 C3 hydrocarbons 8,136 C4 hydrocarbons 171,872 C5 hydrocalbons. '-"fti-f--n 3951 The reduced pressure in separator drum 58 allows low boiling gases such as O2, N2 or CO which are entrained in the liquid phase to be separated therefrom by reason of the reduced pressure and the gases which are thus separated are withdrawn from drum 58 through conduit 14 at the rate of 4,203 pounds per hour and pass through valve63 `randeonduit KY1 4 to drumk 13, as. previously described. If desire`c1,'"because of` the small quantities :of :such r gases present, f or tfor.- other reasons, pressure reduction valve 57 and separator drum 58 may be eliminated'from the process without departing from the scope of our invention. i

yLiquid from s'eparator'drurnwSS is withdrawn through conduit y64 and passes Vvia a pump 66 to a heat exchanger '67 where it is heated lto a temperature offl90 F. From heat exchanger 67 the liquidin conduit 6,4. continues to a fractionator 68 which is a distillation tower. The liquid entering fractionator 68 Vtrom conduit64 has the composition shown in Table In fractionator 68, this liquid is fractionated in order to separate the butane absorbent from the C2-richA product fraction. FractionatorI 68 is maintained at a pressure of 435 p. s. i. g. A liquid -bottoms Yfraction comprising butane absorbent having a density of 4.90 pounds per gallon is -withdrawn from tower 68 through conduit 27 at a temperature of 270 F. at the rate of 175,428 pounds per hour. The liquid in conduit 27 passes through heat exchanger 67 where its temperature is reduced to 166 F. and then is combined with the butane abhorbent in conduit 23 as previouslydescribed. A side stream is withdrawn from fractionator -68-through conduit 69 at a temperature of 265 heated toa temperature of 270 F. in a reboiler 71 and returned to fractionator`68 through conduit 69. Heat for reboiler 71lis supplied by steam which enters through conduit 7`2'and leaves through conduit'73.

An overhead fraction-is lv'vithdrawnlfrom fractionator 68 at a temperature of '127 F. through conduit 74 at the rate of 136,369 pounds per hour and passes througha reftux condenser '764:0 aseparating drum v77 which is operated at a pressure of 420 p. sri. g. and a temperature of 65 F. Liquid from drum A77 is returned as rellnx to fractionator 68 through conduit 78 via pump 79. A C2-rich vapor fraction .is withdrawn 'from drum 77 through conduit 81 and passes from the systemA through a valve 82 as the product of the process. `The vapor withdrawn -through conduit 81 has the composition shown in Table VH.

Other aspects and embodiments of our 'invention as well as modications and alterations of the preferred embodiment described above will become apparent from the above disclosure. Those skilled in the art will recognize that numerous changes and rnodifications, particularly as respects operating conditions and equipment, can be made without departing from the scope of'our invention.

We claim:

`l. The method for separating C2 hydrocarbons from a gaseous feed mixture containing the same and methane which comprises: passing said Vmixture to a rst absorption zone, in said first absorption-zone contacting said mixture with a first absorption medium comprising a compound selected from the group consisting of C3 and C4 ,parafns adapated to preferentially absorb C2 hydrocarbons to form a liquid phase containing said rst absorption medium and C2 hydrocarbons and a vaporous phase containing methane and entrained absorption mediurn, withdrawing said vaporous phase containing entrained absorption medium from said rst absorption zone and passing same to a second absorption zone, in said second absorptionzone contacting said vaporous phase .fromsaid frst absorption zone with a second hydrocarbon absorption medium of lower volatility than ysaid first absorption medium and adapted to preferentially absorb said rst Aabsorption medium to form in said second absorption-zone a vaporous phase and a liquid phase comprising said rst and second absorption media, withdrawing said vaporous and 4liquid phases from `said second absorption zone, from said iirst absorption zone withdrawing the liquid phase containing said first absorption medium and C2 hydrocarbons and passing the` same to a distillation zone to form a vaporous overhead fraction containing C2 hydrocarbons -and a liquid bottoms fraction comprising said first absorption medium, withdrawing said bottoms fraction from said distillation zone and passing the same to said rst absorption zone, and withdrawing said overhead fraction from said distillation zoneas `a product of the process.

2. The method for separating C2 and C3 hydrocarbons from a gaseous feed mixture containing the same and methane which comprises: passing said mixture to a first absorption zone, in said first absorption zone contacting said mixture with a first 'absorption medium comprising a C4 paraffin adapted to preferentially absorb C2 and C3 hydrocarbons to form a liquid phase containing said rst absorption medium and substantially all of said C2 and C3 hydrocarbons and a vaporous phase containing methane and entrained absorption medium, withdrawing said vaporous phase containing entrained absorption medium from said rst absorption zone and passing the sarne to a second absorption zone, in said second absorption zone contacting said vaporous phase from said first absorption Zone with a second hydrocarbon absorption medium of lower volatility than said Vfirst absorption medium and adapted to preferentially absorb said rst absorption medium to form in said second absorption zone a vaporous phase substantially free of said yfirst and second absorption media and a liquid phase comprising said first and second absorption media, withdrawing said vaporous and liquid phases from said second absorptionv zone, from said 4viirst absorption zone withdrawing the liquid phase containing said first absorption medium and substantially all of said C2 and C3 hydrocarbons and passing the same to a distillation zone wherein the liquid phase from said first absorption zone is fractionated into a vaporous overhead yfraction containing substantially all of said C2 and vC3 hydrocarbons and a liquid bottoms fraction comprising said iirst absorption medium, withdrawing said bottoms fractions from said distillation zone and passing the same to said first absorption zone, and withdrawing said overhead fraction from said distillation zone as a product of the process.

3. The process of claim 2 in which the first absorption medium comprises butane.

4. rl'he methgd for separating a fraction rich in C2 and C3 hydrocarbons from a gaseous mixture containing the same, methane and lower boiling components which comprises: passing said mixture to a first absorption zone, in said first absorption zone contacting said mixture with a first absorption medium comprising a C4 paraffin adapted to preferentially absorb C2 and C3 hydrocarbons to form a liquid phase containing said first absorption medium and substantially all of said C2 and C2 hydrocarbons and a vaporous phase containing methane, entrained absorption medium and at least a portion of said lower boiling components, withdrawing said vaporous phase containing entrained absorption medium from said first absorption zone and passing the same to a second absorption zone, in said second absorption zone contactting said vaporous phase from said first absorption zone with a second hydrocarbon absorption medium of lower volatility than said first absorption medium and adapted to preferentially absorb said first absorption medium to form in said second absorption zone a vaporous phase substantially free of said first and second absorption media and a liquid phase comprising said first and second absorption media, withdrawing said vapor and liquid phases from said second absorption zone, from said first absorption zone withdrawing the liquid phase containing said first absorption medium and substantially all of said C2 and C3 hydrocarbons passing the same to a distillation zone wherein the liquid phase from said first absorption zone is fractionated into a vaporous overhead fraction containing substantially all of said C2 and C3 hydrocarbons and a liquid bottoms fraction comprising said first absorption medium, withdrawing said bottoms fraction from said distillation zone and passing the same to said rst absorption zone, and withdrawing said overhead fraction from said distillation zone as a product of the process.

5. The method for separating C2 and C3 hydrocarbons from a gaseous mixture containing the same, methane and lower boiling components which comprises: passing said mixture to a first absorption zone; in said first absorption zone contacting said mixture with a first absorption medium comprising a C4 paraffin adapted to preferentially absorb C2 and C2 hydrocarbons to form a liquid phase containing said first absorption medium and substantially all of said C2 and C3 hydrocarbons, at least a portion of said lower boiling components being entrained in said liquid phase, and a vaporous phase containing methane and entrained absorption medium; withdrawing said vaporous phase containing entrained absorption medium from said first absorption zone andsaid vapor andliquid phases from said second absorption zone; from said first absorption zone withdrawing the liquid phase containing said first absorption medium and substantiallyall of said C2 and C2 hydrocarbons and having at least a portion of said lower boiling components entrained therein and passing the same to a zone of reduced pressure to form therein a vaporous phase comprising said lower boiling components and a liquid phase containing said rst absorption medium and substantially all of said C2 and C3 hydrocarbons; withdrawing from said zone of reduced pressure the vaporous phase thus formed therein; withdrawing from said zone of reduced pressure the liquid phase thus formed therein and passing the same to a distillation zone wherein the liquid phase from said zone of reduced pressure is fractionated into a vaporous overhead fraction containing substantially all of said C2 and C3 hydrocarbons and a liquid bottoms fraction comprising said first absorption medium; withdrawing said bottoms fraction from said distillation zone and passing the same to said first absorption zone; and withdrawing said overhead fraction from said distillation zone as a product of the process.

6. The process according to claim 5 in which the components lower boiling than methane include nitrogen, carbon monoxide, and hydrogen, and the first absorption medium comprises butane.

7. 'Ihe process of claim 1 in which the first absorption medium comprises propane.

8. The process of claim l in which the first absorption medium comprises butane.

References Cited in the file of this patent UNITED STATES PATENTS 2,250,925 Babcock Iuly 29, 1941 2,321,666 Felbeck June 15, 1943 2,765,635 Redcay Oct. 9, 1956 2,771,149 Miller et a1. Nov. 20, 1956 

1. THE METHOD FOR SEPARATING C2 HYDROCARBONS FROM A GASEOUS FEED MIXTURE CONTAINING THE SAME AND METHANE WHICH COMPRISES: PASSING SAID MIXTURE TO A FIRST ABSORPTION ZONE, IN SAID FIRST ABSORPTION ZONE CONTACTING SAID MIXTURE WITH A FIRST ABSORPTION MEDIUM COMPRISING A COMPOUND SELECTED FROM THE GROUP CONSISTING OF C3 AND C4 PARAFFINS ADAPTED TO PREFERENTIALLY ABSORB C2 HYDROCARBONS TO FORM A LIQUID PHASE CONTAINING SAID FIRST ABSORPTION MEDIUM AND C2 HYDROCARBONS AND A VAPOROUS PHASE CONTAINING METHANE AND ENTRAINED ABSORPTION MEDIUM, WITHDRAWING SAID VAPOROUS PHASE CONTAINING ENTRAINED ABSORPTION MEDIUM FROM SAID FIRST ABSORPTION ZONE AND PASSING SAME TO A SECOND ABSORPTION ZONE, IN SAID SECOND ABSORPTION ZONE CONTACTING SAID VAPOROUS PHASE FROM SAID FIRST ABSORPTION ZONE WITH A SECOND HYDROCARBON ABSORPTION MEDIUM OF LOWER VOLATILITY THAN SAID FIRST ABSORPTION MEDIUM AND ADAPTED TO PREFERENTIALLY ABSORB SAID FIRST ABSORPTION MEDIUM TO FORM IN SAID SECOND ABSORPTION ZONE A VAPOROUS PHASE AND A 